The present invention relates to a thickness measuring apparatus and thickness measuring method of measuring the thickness of a semiconductor wafer during execution of wet etching and a wet etching apparatus and wet etching method using the thickness measuring apparatus and method.
In manufacturing semiconductor devices, recently, the necessity of an etching process for thinning a semiconductor wafer with a pattern or the like is increasing.
In such an etching process, a wet etching apparatus for etching a semiconductor wafer while supplying an etchant is used. In a conventional wet etching method, a dummy wafer is etched to confirm the etching rate in advance, and the etching end time is determined on the basis of the etching rate.
However, when such an etching time management method is used, an extra process for etching a dummy wafer is necessary in addition to an actual etching process. In addition, the etching rate may change for each etching process. For this reason, time management assuming a constant etching rate generates a variation in thickness of resultant semiconductor wafers.
To increase the etching accuracy or operation efficiency in the etching process, the thickness of a semiconductor wafer portion must be measured in situ during etching. With this measurement, data of a change in thickness over time during etching can be obtained whereby the end time can be obtained for each etching process. That is, various etching processes can be managed and controlled.
Conventional semiconductor wafer thickness measuring apparatuses include a contact thickness meter and Michelson interference thickness meter. Of these thickness meters, the contact thickness meter cannot be applied to measurement in situ. In addition, since this thickness meter comes into contact with a semiconductor wafer, the wafer may be damaged, and therefore, measurement at a high speed is impossible. If a wafer has a holding substrate or film, the thickness of only the wafer cannot be measured.
On the other hand, the Michelson interference thickness meter measures the thickness of a semiconductor wafer in a noncontact state. As such a thickness meter, an apparatus is disclosed in Japanese Patent Application Laid-Open No. H5-248817. This apparatus irradiates a semiconductor wafer with measurement light and measures a change in thickness over time on the basis of a change in reflection timing of reflected light from the wafer surface. In this case, however, only the position on the upper surface is measured. To obtain the thickness, an initial condition of thickness, such as the position on the lower surface, must be given. Additionally, in a wet etching process using an etchant, since measurement light is reflected by the etchant on the wafer surface, the thickness of the semiconductor wafer cannot be measured.
The present invention has been made to solve the above problems, and has as its object to provide a thickness measuring apparatus and thickness measuring method which can measure the thickness of a semiconductor wafer during execution of wet etching, and a wet etching apparatus and wet etching method using the thickness measuring apparatus and method.
In order to achieve the above object, according to the present invention, there is provided a thickness measuring apparatus for measuring a thickness of a semiconductor wafer during execution of wet etching using an etchant, characterized by comprising (1) a measurement light source which supplies measurement light at each of a plurality of measurement times at a predetermined time interval, (2) light branching means for branching the measurement light from the measurement light source, (3) light output means for outputting one component of the measurement light branched by the light branching means to the semiconductor wafer as a measurement object so as to irradiate the semiconductor wafer from a side of an etching surface to which the etchant is being supplied, (4) light input means for inputting reflected light obtained when the measurement light irradiated from the light output means is reflected by the etchant or semiconductor wafer, (5) reference light generating means for passing the other component of the measurement light branched by the light branching means through a reference optical path designed to be able to change an optical path length so as to generate reference light for which a reference optical path length is set, (6) light coupling means for obtaining interference light by coupling the reflected light from the light input means to the reference light from the reference light generating means, (7) photodetection means for detecting the interference light from the light coupling means, (8) raw thickness value calculating means for calculating a raw thickness value of the semiconductor wafer on the basis of an optical path length difference in reference optical path length between two light intensity peaks selected from a plurality of light intensity peaks each having a light intensity more than a set threshold value, using a light intensity distribution representing a correlation between the reference optical path length set by the reference light generating means and a light intensity of the interference light detected by the photodetection means at each of the measurement times, and (9) statistical thickness value calculating means for, at each of the measurement times after an elapse of a specified time from a initial measurement time, determining a thickness change line by linear approximation for a time-rate change in a plurality of valid raw thickness values within a set allowable numerical value range so as to calculate a statistical thickness value from the thickness change line, wherein (10) at the first measurement time after the elapse of the specified time from the initial measurement time, the statistical thickness value calculating means executes, for the time-rate change in valid raw thickness value before the measurement time, data sorting calculation including determination of a thickness change line for sorting by linear approximation, setting of a sorting numerical value range for the thickness change line for sorting, and sorting of data for which the raw thickness value outside the sorting numerical value range is to be invalidated, then determines the thickness change line by linear approximation for the time-rate change in valid raw thickness value after sorting, and sets the allowable numerical value range from the thickness change line.
According to the present invention, there is provided a thickness measuring method of measuring a thickness of a semiconductor wafer during execution of wet etching using an etchant, characterized by comprising (1) the measurement light supply step of supplying measurement light from a measurement light source at each of a plurality of measurement times at a predetermined time interval, (2) the light branching step of branching the measurement light from the measurement light source, (3) the light output step of outputting one component of the measurement light branched in the light branching step to the semiconductor wafer as a measurement object so as to irradiate the semiconductor wafer from a side of an etching surface to which the etchant is being supplied, (4) the light input step of inputting reflected light obtained when the measurement light irradiated in the light output step is reflected by the etchant or semiconductor wafer, (5) the reference light generating step of passing the other component of the measurement light branched in the light branching step through a reference optical path designed to be able to change an optical path length so as to generate reference light for which a reference optical path length is set, (6) the light coupling step of obtaining interference light by coupling the reflected light input in the light input step to the reference light generated in the reference light generating step, (7) the photodetection step of detecting the interference light coupled in the light coupling step, (8) the raw thickness value calculating step of calculating a raw thickness value of the semiconductor wafer on the basis of an optical path length difference in reference optical path length between two light intensity peaks selected from a plurality of light intensity peaks each having a light intensity more than a set threshold value, using a light intensity distribution representing a correlation between the reference optical path length set in the reference light generating step and a light intensity of the interference light detected in the photodetection step at each of the measurement times, and (9) the statistical thickness value calculating step of, at each of the measurement times after an elapse of a specified time from a initial measurement time, determining a thickness change line by linear approximation for a time-rate change in a plurality of valid raw thickness values within a set allowable numerical value range so as to calculate a statistical thickness value from the thickness change line, wherein (10) in the statistical thickness value calculating step, at the first measurement time after the elapse of the specified time from the initial measurement time, (11) for the time-rate change in valid raw thickness value before the measurement time, data sorting calculation including determination of a thickness change line for sorting by linear approximation, setting of a sorting numerical value range for the thickness change line for sorting, and sorting of data for which the raw thickness value outside the sorting numerical value range is to be invalidated is executed, then the thickness change line by linear approximation is determined for the time-rate change in valid raw thickness value after sorting, and the allowable numerical value range from the thickness change line is set.
In the above-described thickness measuring apparatus and thickness measuring method, reflected light obtained when the semiconductor wafer is irradiated with measurement light, and the measurement light is reflected is coupled to reference light that is branched from the measurement light and passes through a predetermined optical path to set a reference optical path length with respect to the optical path length of the reflected light, and the resultant interference light is detected. The thickness of the semiconductor wafer during wet etching is measured from a plurality of light intensity peaks generated in the light intensity distribution of the interference light.
At this time, the measurement light with which the semiconductor wafer is irradiated is reflected by the etchant surface and the upper surface (etching surface) and lower surface of the semiconductor wafer. Light intensity peaks corresponding to the surfaces are obtained in the light intensity distribution. Hence, when two light intensity peaks corresponding to the upper and lower surfaces of the semiconductor wafer, which are selected on the basis of a predetermined selection criterion, are used, the thickness of the semiconductor wafer or a time-rate change in thickness can be measured during wet etching independently of the presence of the etchant. In addition, instead of obtaining the thickness from the reflected light from the wafer upper surface and initial conditions as a reference, reflected light from both the upper and lower surfaces of the wafer is used. For this reason, even when the state of the semiconductor wafer or etchant changes, the thickness of the semiconductor wafer can always be accurately measured.
In thickness measurement after a sufficient number of raw thickness value data are obtained since the elapse of a specified time given in advance, fitting calculation for linearly approximating the time-rate change is executed, and a statistical thickness value is calculated from a resultant thickness change line. For this reason, the influence of a statistical variation in the raw thickness value can be reduced. In thickness measurement, a variation may be generated due to a measurement error when, e.g., the light intensity peak from the etchant surface is not detected, independently of the statistical variation. To cope with this, the raw thickness value to be used to determine the thickness change line is limited to a raw thickness value within a predetermined allowable numerical value range. Raw thickness values with measurement errors can be excluded, and the influence of error variations can be reduced.
When the thickness change line is to be determined for the first time after the elapse of the specified time, statistical processing for raw thickness value data is executed by fitting calculation for data sorting and fitting calculation for determination of the thickness change line. The raw thickness value sorted by statistical processing at the first measurement time after the elapse of the specified time and the determined thickness change line are initial conditions of statistical processing at each of the second and subsequent measurement times.
Hence, after preliminary linear approximation for data sorting is executed, and raw thickness values with measurement errors are invalidated using the thickness change line for sorting and sorting numerical value range and excluded from the statistical processing, statistical processing including linear approximation is executed again to set the thickness change line and allowable numerical value range. With this arrangement, at each measurement time after the elapse of the specified time, the influence of an error variation and the like can be efficiently reduced. Data sorting calculation using the thickness change line for sorting may be executed not once but repeatedly a plurality of number of times to more reliably exclude excess raw thickness value data such as raw thickness values with measurement errors.
According to the wet etching apparatus and method using such a thickness measuring apparatus and method, on the basis of the thickness value obtained for the semiconductor wafer during wet etching, the end of wet etching by stopping supply of the etchant, or change of the etching rate can be appropriately controlled through the etching control means.